Refactor AnselLayout.

True ondemand content loading, correct handling of border values
during image scaling, bunch more.
Let AnselLayout manage adding the initial row.
Finish on demand content loading, improve some logic.
Save these as Element objects, simplify.
Tweak this label.

Make it more obvious this is a stream of all photos.
Pass the perPage option from AnselCore.
Don't overwrite the last row, add a new one.
Make sure we fire the event on the correct element.
ws
Fix callback name.
These are handled elsewhere.
Fire AnselLayout:scroll when we know we have more vertical space.
Check for available vertical space and fetch more images if needed.
This seems to work better.
Fix grabbing the correct gallery keyImage.
ws
Remove empty rows.

Rows may be left over after the grid is resized.
Add @todo about why this fails.
Fix conditional.
Add comment about why we need to do this.